DESCRIPTION: (Adapted From The Investigator s Abstract): Neuropsychological theory has it that the occurrence of errors in routine action involving objects is diagnostic of ideational apraxia - a left hemisphere syndrome. A large-scale study conducted under the current grant is showing that vulnerability to action errors exists in both let and right hemisphere strokes and in closed head injury. Moreover, the three patient groups display a similar response to situational complexity and a similar pattern of errors. To elucidate the conditions that bring about action breakdown, a program of single case studies is being conducted. These studies suggest that the status of executive functions is more important than praxis or semantic memory for objects. This continuation grant aims at a better understanding of the necessary and sufficient conditions for routine action breakdown. The formal study of naturalistic action in brain damaged populations and the program of single case studies begun under the current grant will be continued, and two new series of experiments will be undertaken. The first is an exploration of distracter effects in reaching to targets, which bears on the mechanisms underlying objects substitution errors. The second explores whether competing plans or concurrent memory-load manipulations induce action errors in non-neurological subjects and in patients who are vulnerable to attention/working memory deficits. This second series of experiments is motivated by a theory of attention and action proposed by Norman and Shallice. Shallice and colleagues recently developed a computational model that implements aspects of the theory relating to routine action production. Under a subcontract to Shallice's group, this modeling effort will continue and will focus on simulating normal and pathological performance under increasing situational complexity. Finally, having developed methods for assessing naturalistic action production under increasing task demands, we now propose to create a screening tool for use in the clinic and to test its predictive validity against scales of instrumental activities of daily living.